1. Field of the Invention
The present invention relates to a resin coated carrier used in an electrophotographic image forming apparatus for developing an electrostatic latent image formed on an image bearing member to visualize, a two-component developer including the resin coated carrier, a developing device using the two-component developer and an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses employing an electrophotographic technology such as printers and copiers use, as a developer for developing an electrostatic latent image formed on an image bearing member to form a visible image, a two-component developer consisting of a toner and a carrier or a one-component developer consisting of a toner alone, for example. A magnetic brush development method that uses the two-component developer among them is excellent in image quality and high-speed printing compared to other development methods, and is therefore widely used.
An image forming apparatus using the magnetic brush development method is provided with a developer bearing member comprising, for example, a cylindrical-shaped metal sleeve and a magnet roller provided inside the metal sleeve, in which permanent magnets are provided as a magnetic field generating section with a N pole and a S pole arranged alternately. By causing the surface of the metal sleeve of the developer bearing member to carry the two-component developer and rotating only the metal sleeve with the magnet roller fixed, it is possible to transport the two-component developer to a development area that faces an image bearing member on which an electrostatic latent image has been formed. A development electric field applied between the developer bearing member and the image bearing member causes only a charged toner to electrostatically attach to the image bearing member to form a visible image.
The toner in the two-component developer is mixed and stirred with the carrier in a developing unit including the developer bearing member so as to be charged through contact friction. In dry two-component development, an electrostatic force of the frictionally charged toner is used for electrically handling to form a visible image on the image bearing member, and it is therefore important to control a charge amount of the toner. Although the charge amount of the toner varies depending on various conditions in a system, it is desirable that a value of the charge amount of the toner is stabilized for stability of the system.
Furthermore, high-quality and high-speed printing tends to be considered as important in copiers and printers in recent years. In this case, charge stability of the developer becomes particularly important. To achieve formation of a high-quality image, it is necessary to arrange a determined amount of the toner to a determined place. In the electrophotographic method, handling of the toner is performed by an electrostatic force, and the toner is required to maintain high charge at a certain level or more so that the toner overpowers other external forces such as an adhesive force to be transported by an electric field. In addition, since the number of print sheets is increased with increase in speed of an image forming apparatus, it is strongly demanded to reduce the number of times or the labor of the maintenance and a developer that operates stably over a long term is desired.
For such a demand, the carrier needs to frictionally charge the toner to a desirable polarity and to a desirable charge amount over a long term constantly. The carrier generally used in the two-component developer for charging the toner is different from the toner and stays in the developing unit for a long term, it is concerned that the charge applying capability is reduced due to toner spent or stress from stirring and mixing. In addition, there is also a concern that toner scattering resulting therefrom causes contamination in an image forming apparatus. Accordingly, a highly-durable carrier that withstands degradation over time and that is capable of stably maintaining the charge amount of the toner over a long term is required.
In order to meet such a demand for the carrier, a resin coated carrier having the carrier surface coated with a resin has been proposed. Specific examples thereof include a resin coated carrier having the carrier surface coated with a styrene-acrylic copolymer resin or a polyurethane resin each having high surface energy, and a resin coated carrier having the carrier surface coated with a fluorine resin having low surface energy. The styrene-acrylic copolymer resin and the polyurethane resin having high surface energy have great adhesiveness to a carrier core, but have a drawback that the toner is easily spent, whereas the fluorine resin having low surface energy has poor adhesiveness to a carrier core although being effective against the toner-spent, and therefore, there is a drawback that the resin coating layer separates from the carrier core in stirring in the developing tank and stable charging is inhibited.
For the purpose of improvement of those drawbacks, a resin coated carrier using two or more of resins having different molecular weight in coating a carrier with a resin is proposed to increase durability of a resin coating layer and adhesion to a carrier core. For example, Japanese Unexamined Patent Publication JP-A 7-72666 (1995) discloses an electrophotographic carrier comprising a carrier core coated with a resin, wherein the coating resin has a weight average molecular weight of 50,000 to 250,000 and has a molecular weight distribution measured by Gel Permeation Chromatography (abbreviated as GPC) such that the maximum value of a low molecular weight component appears in a weight molecular weight region of 10,000 to 60,000 and the maximum value of a high molecular weight component appears in a weight molecular weight region of 100,000 to 300,000.
Japanese Unexamined Patent Publication JP-A 2002-14493 discloses an electrophotographic carrier comprising a core comprising at least a magnetic material, and at least a silicone resin coating layer formed on the surface of the core, wherein the silicone resin is prepared by crosslinking a silicone compound having two or more molecular weight peaks on the basis of weight.
Japanese Unexamined Patent Publication JP-A 2007-33568 discloses an electrophotographic developing carrier comprising magnetic particles containing soft ferrite, a low molecular silicone resin layer having a weight average molecular weight of 300 to 1,000 formed on the surface of the magnetic particles, and a high molecular silicone resin layer having a weight average molecular weight of 5,000 or more formed on the surface of the low molecular silicone resin layer.
In recent years, a full color image forming apparatus is advanced, and with the advancement, improvement in a toner is actively conducted. Improvement in a wax resin and an external additive of a toner is part of the improvement. A toner used in oilless fixing contains a low-boiling wax resin dispersed therein in order to ensure fixing releasability in the toner itself. When thermal stress is applied to a two-component developer containing such a toner, a wax oozes on the surface of a toner, resulting in contamination of the surface of a carrier. As a result, charge performance of a developer to a toner is decreased, and additionally, blocking of a developer is induced. When decrease in charge performance to a toner and blocking occur, image quality is deteriorated, and life of a developer is extremely shortened.
An external additive of a toner imparts fluidity to a toner and further has a function as a control aid of a charging amount of a toner. In a full color image forming apparatus, an external additive having a large particle size, specifically an external additive having an average primary particle size of 50 nm or more, tends to be added for the purpose of increasing transfer efficiency of a toner. However, when a developer comprising a toner having an external additive having a large particle size added thereto, and a carrier is used over a long period of time, the external additive having a large particle size easily adheres to the surface of the carrier and accumulates thereon. As a result, normal frictional charging between a toner and a carrier is prevented by adhesion of an external additive, and charge imparting ability of a carrier is decreased. For this reason, in the case of using a toner comprising a two-component developer and an external additive having a large particle size added thereto, it is difficult to stably charge a toner over a long period of time.
Against those problems, the electrophotographic carrier disclosed in JP-A 7-72666 uses an acrylic resin having relatively high surface energy as a resin coating layer. As a result, blocking of a developer is generated by the use of such a carrier over a long period of time, and the feeding amount of a developer is decreased.
Further, the electrophotographic carriers disclosed in JP-A 2002-14493 and JP-A 2007-33568 are that the surface of a resin coating layer is difficult to be scraped off. For this reason, in the case of using the electrophotographic carriers disclosed in JP-A 2002-14493 and JP-A 2007-33568 together with a toner having added thereto an external additive having a large particle size, the external additive having a large particle size accumulates on the surface of a resin coating layer due to the use over a long period of time. As a result, charge imparting ability of a carrier is decreased, and charge stability of a toner is decreased.